The formation of neuromuscular synapses requires a mutual exchange of signals between motor neurons and muscle fibers leading to the formation of a highly specialized postsynaptic membrane and a highly differentiated nerve terminal, which insure that synaptic transmission is fast, robust and reliable. We found that Lrp4 has a key role in this exchange, as it acts bi- directionally to coordinate neuromuscular synapse formation. Lrp4 not only binds neuronal Agrin, activating MuSK and stimulating postsynaptic differentiation, but also functions as a direct muscle-derived retrograde signal that is necessary and sufficient for presynaptic differentiation. How Lrp4 stimulates presynaptic differentiation is not understood. Here, we propose to identify the motor neuron receptor for Lrp4 as a first step to understand how Lrp4 induces the differentiation of motor nerve terminals. Our previous studies showed that Lrp4 induces the clustering of synaptic vesicle and active zone proteins, thereby organizing the machinery for release of neurotransmitter. Retrograde signals, however, also cause motor axons to terminate and form synapses: in the absence of Lrp4 or MuSK, motor axons fail to stop and instead grow throughout the muscle without forming synapses. Here, we propose to determine whether Lrp4 is itself a 'stop' signal that arrests motor axon growth or whether Lrp4-mediated activation of MuSK leads to the production of a novel signal that control motor axon growth. The experiments described here should provide new insights into the mechanisms that control presynaptic differentiation during development and maintain neuromuscular synapses in adults. As such, these studies are likely to shed new light into neuromuscular diseases, including amyotrophic lateral sclerosis, congenital myasthenia, myasthenia gravis and age-related muscle wasting, sacropenia.